Design of Steel Structures Design of Towers and Masts
INTRODUCTION
A tower or mast is a tall skeleton structure with a relativel small cross!section" which has a large ratio #etween height and ma$imum width%
A tower is a freel standing self su&&orting structure fi$ed to the #ase or foundation while a mast is tall structure" &inned to the #ase of foundation and #raced with gus etc%
A''(ICATION A''(ICAT ION ) US* i%
*lectric &ower transmission +,- to ./ m high0
ii%
Microwave transmission for communication
iii%
Radio transmission +short and medium wave wireless0
iv%% iv
Television transmission +,-- m to 1-- m0
v%
Satellite rece&tion
vi%
Air traffic control
vii%
2lood light stand +,/ to /- m0
viii%
Meteorological measurements
i
%$Derrick and crawler cranes
%$Oil drilling masts%
$i%
Over head tanks%
INTRODUCTION
A tower or mast is a tall skeleton structure with a relativel small cross!section" which has a large ratio #etween height and ma$imum width%
A tower is a freel standing self su&&orting structure fi$ed to the #ase or foundation while a mast is tall structure" &inned to the #ase of foundation and #raced with gus etc%
A''(ICATION A''(ICAT ION ) US* i%
*lectric &ower transmission +,- to ./ m high0
ii%
Microwave transmission for communication
iii%
Radio transmission +short and medium wave wireless0
iv%% iv
Television transmission +,-- m to 1-- m0
v%
Satellite rece&tion
vi%
Air traffic control
vii%
2lood light stand +,/ to /- m0
viii%
Meteorological measurements
i
%$Derrick and crawler cranes
%$Oil drilling masts%
$i%
Over head tanks%
INTRODUCTION C(ASSI2ICATION De&ending u&on the si3e and t&e of loading" towers are grou&ed into two heads4 +a0
Towers with large vertical loads5 +such as those of over head water tanks" oil tanks" meteorological towers etc%0 have their sides made u& of vertical or inclined trusses%
+#0
Towers with mainl hori3ontal wind loads5 su#6ected &redominantl to wind loads categori3ed as4 i%
Self!su&&orting towers or 2ree standing towers or (attice towers
2ree standing towers" known as lattice towers" are generall s7uare in &lan and are su&&orted # four legs" fi$ed to the #ase% These towers act as vertical cantilever trusses" su#6ected to wind and8or seismic loads% 2ree standing towers are commonl used for T% 9% microwave transmission" &ower transmission" flood light holding etc% ii%
:ued towers or Masts
gued towers are hinged to the #ase" and are su&&orted # gu wires attached to it at various levels" to transmit the wind forces to the ground% Due to this reason" gued tower of the same height is much lighter than a self!su&&orting tower% ;owever" it re7uires much larger s&ace in &lan" to accommodate the &lacement of gu ro&es%
2ig% , T&ical free standing towers
2ig% < :ued tower or Mast
(ATTIC* TO=*RS CON2I:URATIONS AND >RACIN: S?ST*MS
The self su&&orting towers" su#6ected &redominantl to wind loads" are called lattice towers.
Such towers are s7uare or rectangular in &lan% The width # of the side face at the #ase ma var #etween ,8@ to ,8,< of the height of tower%
The to& width of towers is ke&t #etween ,%/ to 1 m or more" de&ending u&on the re7uirement%
Some common configurations with #racing sstems are listed as4
i%
Single diagonal #racings +2ig <,%1a04 This is the sim&lest form of #racing% The wind shear at an level is shared # the single diagonal of the &anel" such #racing is used for towers u& to 1- m height%
ii%
! #racing +2ig <,%1#04 !This is a dou#le diagonal sstem without hori3ontal #racing" and used Bfor towers u& to /- m height% It is a staticall determinate structure%
iii%
!> #racing +2ig <,%1c04 This is a dou#le diagonal sstem with hori3ontal #racings" such #racings are 7uite rigid" and ma #e used for towers u& to /- m height% The structure is staticall indeterminate% The hori3ontal mem#ers are redundant mem#ers and carr onl nominal stresses%
(ATTIC* TO=*RS CON2I:URATIONS AND >RACIN: S?ST*MS iv%
#racing +2ig <,%1d04 Such #racing gives large head room% The structure is staticall determinate% Such #racing can #e used for towers of /- to <-- m height%
v%
!>! #racing +2ig <,%1e04 This is com#ination of and > #racing where hori3ontal mem#ers are &rovided onl at the level of crossing of diagonals% The structure is staticall indeterminate% The sstem is suita#le for towers /- to <--m height%
vi%
= #racing +2ig <,%1f04 This sstem uses a num#er of overla&&ing diagonals% The structure is staticall indeterminate% ;owever" the effective length of the diagonals is reduced% The sstem is 7uite rigid and ma #e used for towers of /to <-- m height%
vii%
?! #racing 2ig <,%1 +g04 This sstem gives larger head room and can #e used for lower &anels% The sstem is staticall determinate%
viii%
Arch #racing 2ig% <,%1+h04 Such a #racing can #e ado&ted for wider &anels% This sstem also &rovides greater head room% The sstem is staticall determinate%
i
%$Su#divided 9 #racing 2ig <,%1+i04 Such #racing are used for tall communication sstems towers" radio and T9 transmission etc%" for heights #etween /- to <--m%
%$Diamond lattice sstem4 2ig <,%1+604 A t&ical diamond lattice sstem used for towers of ,-- to <-- m height% The #ase width is ke&t at ,8/ to ,8@ of the height%
(ATTIC* TO=*RS (OADS ACTIN: ON TO=*RS 2ollowing are the various t&es of loads acting on a lattice tower4 a0
#0
:ravit loads +W g0 i%
=eight of mem#ers
ii%
=eight of &latforms" railings" ladders" lifts etc%
iii%
=eight of antenna" instruments" a&&liances etc%
iv%
=eights of gussets and secondar #racings
v%
(ive loads
(ateral loads i%
=ind load
ii%
Seismic loads
c0
*rection loads
)
The gravit loads are almost fi$ed" since these are de&endent on the structural design" Seismic load is also not critical as mass of the structure is not ver heav and it is more near the ground% ;owever" ma$imum wind &ressure is the chief criterion for the design of lattice towers%
(ATTIC* TO=*RS CA(CU(ATIONS
2OR =IND (OAD +IS E/ 'art III0
The designed wind s&eed V z +m8s0 is given # 93 F =here"
9> 4 #asic wind s&eed in at ,- m height 4 &ro#a#ilit factor
4 terrain" height and structure si3e factor 4 to&ogra&h factor" the value of which varies from , to ,%. The designed wind &ressure pz +N8m<0 is given #
The wind force on an mem#er is given #
=here"
4 effective frontal area
4 net wind force coeff% =hich de&ends on solidit ratio G of the tower G 4 Solidit ratio F o#struction area of the front face8gross area of face
2OR TO=*RS COM'OS*D O2 2(AT SID*D M*M>*RS
2OR SUAR* TO=*RS COM'OS*D O2 ROUND M*M>*RS
2OR TRIAN:U(AR TO=*RS COM'OS*D O2 ROUND M*M>*RS
(ATTIC* TO=*RS
Tower A&&urtenances4
The wind loading on tower a&&urtenances" such as ladders" conduits" lights" elevators etc% shall #e calculated using a&&ro&riate net &ressure coefficients for these elements%
Tower mountings4
Usuall" towers have mountings such as antenna dishes etc% on these mountings can #e com&uted # suita#l selecting &ressure coefficient% The values of Cf for some limited sha&es are given as4 9A(U*S O2 2ORC* CO*22ICI*NT C f 2OR SO(ID S;A'*S O2 MOUNTIN:
(ATTIC* TO=*RS ANA(?SIS
AND D*SI:N
The wind loads" acting at &anels &oints have two effects i%
;ori3ontal shear effect due to lateral load
ii%
9ertical force due to moments due to lateral load
)
The lateral load due to wind is resisted mainl # the we# mem#ers while the gravit loads and the vertical force due to wind moments are resisted # chords or leg mem#ers%
)
At an level under consideration" let =g #e the gravit load and Mw #e the moment due to lateral loads% Then force 2l due to lateral loads is given #
2or a s7uare #ase tower4 2or a triangular #ase tower4 2or a multi J &ost tower4 )
Similarl" if K is the inclination of the tower leg with the a$is of the tower" the force due to gravit loads is given #
" where" N F no% of legs in tower
(ATTIC* TO=*RS ANA(?SIS
AND D*SI:N
;ence the total force 2 in the leg is given #
2F
The lateral load +i%e% wind shear0 is resisted # the we# mem#er in tension at the section%
The leg mem#ers are designed as com&ression mem#ers while the we# mem#ers as tension mem#ers%
The width of #ase is taken e7ual to ,8 to ,8,< of the height" while the inclination or &itch of the sides is ke&t #etween ,8,@ to ,8.-%
(ATTIC* TO=*RS *AM'(* A @- m high microwave lattice tower is to #e #uilt near Agra where the terrain at the site is nearl level ground with terrain of categor <% The diameter of the hemi! s&herical antenna disc fi$ed at the to& is 1 m% The width of the tower at the to& has to #e 1%/ m% Select a suita#le configuration for the tower and determine ma$imum com&ressive force and tension in the tower legs and the ma$imum shear at the #ase" for the following data4 =eight of antenna disc and fi$tures 4 L kN =eight of &latform at to& 4 -%< kN8m< =eight of railing at to& 4 -%1- kN8m < =eight of ladder and the cage 4 -%@/ kN8m =eight of miscellaneous items 4 <%/ kN +such as #eacon lights" lightening ca#les etc%0
(ATTIC* TO=*RS SO(UTION Selection of tower configurations ee& >8; ratio as ,8" ;ence" >ase width > F @-8 F E%/ ee& to& ,< m &ortion &erfectl straight +vertical0" and remaining &ortion inclined% (et us kee& @ &anels in this to& height of ,< m so that length of leg mem#er in this &ortion F < m% Inclination of #ase legs F tan !, +E%/!1%/08+<+@-!,<00 F<%1/Lo or ,8<. Divide the com&lete height in four segments as shown in 2ig% . 'rovide t&e #racing% Reduce the length the diagonals in the #ottom segment # &roviding secondar #racings% The width # of the tower at various heights will #e as under4 #o +at - m a#ove #ase0 F E%/ m #L +at L m a#ove #ase0 F @%E/ m #, +at , m a#ove #ase0 F @ m #
2ig% / (attice tower
(ATTIC* TO=*RS SO(UTION Selection of tower configurations The inclination of diagonals at various heights will #e as under4 Segment I
4
K@- F K. F tan!, <8+1%/8<0 F .%,o
Segment II
4
K.-%/ F tan!, <%/8+.%,
Segment III
4
K
Segment I9
4
KL F tan!, .%/8+@%E/8<0 F /1%,1o
Computation of gravity loads at base =eight Of railing F -%1 $ . F .%< kN =eight of &latform F +-%< $ 1%/ $ 1%/0 F ,-%-/ kN (ive load E/-N8m F +-%E/ $ 1%/ $ 1%/0 F L%,L kN =eight of ladder" cage etc% F -%@/ $ @- F 1L kN Assume the self weight of tower truss .%/ kN8m height" then Self weight of tower truss F .%/ $ @- F
(ATTIC* TO=*RS SO(UTION Computation of wind loads >asic wind s&eed F .E m8s +for Agra0" k, F ,%-E5 k1 F ,%- +&lain ground0% The structure is of class C" and terrain is of categor <% ;ence from code" k< -%LL" ,%-/" ,%-L and ,%,, res&ectivel for ," 11" . and @- m heights% ;ence" the design wind s&eed at various height are as under4 9,D F ,%-E Q ,%- Q -%LL Q .E F .L%EL m8s 911 F ,%-E Q ,%- Q ,%-/ Q .E F /<%- m8s 9.D F ,%-E Q ,%- Q ,%-L Q .E F /.%< m8s 9@- F ,%-E Q ,%- Q ,%,, Q .E F //%< m8s The design wind &ressures at various heights are as under4 &,D F -%@+.L%EL0< Q ,-!1 F ,%.E kN8m< &11 F -%@+/<%-0< Q ,-!1 F ,%@E1 kN8m< &.D F -%@+/.%<0< Q ,-!1 F ,%-1 kN8m< &,D F -%@+//%<0< Q ,-!1 F ,%E- kN8m<
(ATTIC* TO=*RS SO(UTION Computation of wind loads The average frontal area for various segments are as under4 AI F 1%/ Q ,< F .< m < AII F @,%E/ m< AIII F -%@
(ATTIC* TO=*RS SO(UTION The lateral loads at various segments will #e as under4 Segment I
4
2(I F A Cf &3 F +.< $ -%
Segment II
4
2(II F 1%-- kN
Segment III
4
2(III F L.%L@ kN
Segment I9
4
2(I9 F ,,L%<. kN
Antenna 4
2(A F ,%/, kN
Total load"
2( F 1,/%E, F 1,@ kN
the moments of lateral loads at the #ase will #e given as MI F 2(I $ $ I F /L%L $ $ /. F ./E1%E kNm MII F .E-L%. kNm MIII F 11/%. kNm MI9 F ,.@,%/ kNm MAntenna F EL1%, kNm Total wind moment F,.L<1 kNm 2orce in each leg" 2l F F ,.-E kN Ma$% com&ressive force in each leg F ,.-E P F ,.L/ kN Ma$% u&lift at the #ase F ,.-E ! F ,1,L kN Ma$% transverse shear F 1,@ kN
TRANSMISSION (IN* TO=*RS
Transmission line towers are used for su&&orting the e$tra high voltage +*;A0 electric transmission lines% Due to ver heav currents these transmission lines should #e carried at a higher level from the ground level%
2ollowing are various t&es of structures which su&&ort the electric &ower transmission lines4
a0
Structures made of tim#er
#0
c0
i%
=ood &oles
ii%
=ood ;!&oles
Structures made Of concrete i%
R%C%C% &oles
ii%
're!stressed concrete &oles
Structures made of structural steel i%
Round or I!section steel &oles
ii%
2a#ricated steel &oles
iii%
2le$i#le towers
iv%
Semi!fle$i#le towers
v%
Self!su&&orting wide #ase towers
vi%
:ued towers%
TRANSMISSION (IN* TO=*RS CON2I:URATIONS
2ig% . shows various configurations of self!su&&orting wide #ase towers% The main #racing sstem ma #e of three t&es4 i%
Tension sstem5 in this sstem" the diagonal mem#ers have l8r ratio high enough to act in tension onl" the #ecome dumm when su#6ected to com&ression%
ii%
Tension!com&ression sstem5 is suita#le where lateral dimensions of the tower are not too large with res&ect to the tower loads%
iii%
#raced sstem5 is suita#le onl for large towers%
)
The we# &atterns are so chosen that tension mem#ers are long and com&ression mem#ers are short and the inclination of mem#ers ma #e #etween .- o to @-o %
)
De&ending on the voltage rating +@@ to /-- k90 and the num#er of circuits" the height of transmission line tower varies from <- to .- m and the length of cross! arm varies from . to , m% the ratio of width >" to height ;" is ke&t at ,8@ for tangent and small angle towers" ,8/ for medium angle towers and ,8. for large angle towers%
)
The economical #ase width > is &ro&ortional to the s7uare root of the moment" and is e$&ressed # >F
where"
M is over turning moment
is const% and varies from -%-L to -%,@
TRANSMISSION (IN* TO=*RS (OADS ON TO=*R The transmission line towers are su#6ected to the following loads4 a0
#0
9ertical loads i%
=eight of tower structure
ii%
=eight of insulator strings and fittings
iii%
=eight of &ower conductors
iv%
=eight of ground wire
v%
=eight of ice coatings +if an0
vi%
=eight of maintenance crew +line man0 with tools +,%/ kN0
(ateral or hori3ontal loads i%
=ind +or seismic0 load on conductors
ii%
=ind +or seismic0 load on ground wire
iii%
=ind +or seismic0 load on insulator string
iv%
=ind +or seismic0 load on tower structure
v%
Transverse com&onents of tensions in conductors and earth wire
TRANSMISSION (IN* TO=*RS (OADS ON TO=*R c0
d0
(ongitudinal loads +'0 i%
Un#alanced &ull due to a #roken conductor
ii%
Un#alanced &ull due to #roken ground wire
iii%
Seismic load on wires
iv%
Seismic load on tower structure
v%
(oad due to tem&erature variation
Torsional +Mt0 i%
Due earth wire #roken
ii%
Due to conductor #roken
Conditions of design Design is done under two conditions4 iii%
Normal condition
iv%
>roken wire Condition5 A #roken wire condition occurs when a wire +(e! conductor wire or earth wire0 #reaks from one line" giving rise to an un#alanced longitudinal force%
As &er IS 4 -< +'art ,0" the following #roken wire conditions ma #e assumed in design%
TRANSMISSION (IN* TO=*RS D*SI:N S'AN The following terminolog is used for various t&es of s&an4 i%
Normal s&an4 It is the centre to centre distance #etween towers%
ii%
=ind s&an4 The wind s&an +or wind load s&an0 is the sum of the two half s&ans ad6acent to the su&&ort under consideration%
iii%
=eight s&an4 the weight s&an +or vertical load s&an0 is the hori3ontal distance #etween the lowest &oints of the conductor" on the two s&ans ad6acent to the tower%
TRANSMISSION (IN* TO=*RS =eight
of tower
The weight +=0 Of tower ma #e estimated # com&arison with similar e$isting towers% Alternativel" it ma also #e estimated with the hel& of the following formulae # Rle4 =F; =here"
kN
; is overall height of tower a#ove ground +m0
M is overturning moment at ground" due to wind" in kN!m k constant" the value of which usuall lies #etween -%-1/ and -%-.@
=eight of conductors and ground wire
The vertical load due to conductors and ground wire shall #e #ased on the a&&ro&riate weight s&an% A &rovision of ,%/ kN ma #e made for the weight of a lineman%
In com&uting the weight of conductor and earth wire" the weight s&an" which is ,%/ times the normal s&an or wind s&an" is used%
In #roken wire condition" @- of the weight s&an is used" accounting for ,- for the #roken wire and /- for the s&an with un#roken wire%
2or tower and cross!arm design" the weight of maintenance crew +,%/ kN0 is used for cross!arm design onl" an additional errection load of 1%/ kN is used%
The weight of string insulator" <
9*RTICA( (OADS DU* TO CONDUCTOR AND *ART; =IR*
TRANSMISSION (IN* TO=*RS (ateral loads due to wind i.
Wind load on tower structure4 =ind &ressures on towers and su&&orts shall #e com&uted as &er IS 4 E/ ! ,LE% The wind load is then com&uted # multi&ling the #asic wind &ressure # the e$&osed &ro6ected area" using a&&ro&riate solidit ratio and wind force coefficient% In case of lattice steel and other com&ound structures" the wind &ressure on the leeward side mem#ers ma #e taken as one! half the &ressure on wind ward side mem#ers% The wind &ressure intensit on towers varies from ,%/ to <% kN8m<" de&ending on the 3one and the height a#ove the ground where wind is considered%
ii.
Wind load on insulator strings4 In calculating the wind &ressure on insulator strings +if an0" the &ressures as for towers are to #e used on -%/ times the &ro6ected area of the clinder having its diameter e7ual to the diameter of the insulator skirt%
iii.
Wind load on conductors and earth wire4 The wind &ressure on wires ma #e taken as -%.< kN8m< for light wind 3one" -%.. kN8m <4 for medium wind 3one and -%/, kN8m< on heav wind 3one% 2or wind load calculations" the full &ro6ected area of each wire +whether single or a &art of #undle of wires0 over a length of wind s&an is used% Thus" the sha&e factor of ,%- is taken for circular wires% The wind s&an +(B0 is taken as the sum of the half s&ans on either side of su&&ort under consideration%
TRANSMISSION (IN* TO=*RS (ateral loads due deviation K In addition to the lateral load due to wind" lateral +or hori3ontal0 load is also induced due to deviation in the line wires% Thus if T is the tension in the wire" the lateral load due to deviation in the direction will #e e7ual to
TRANSMISSION (IN* TO=*RS (ongitudinal loads4
(ongitudinal loads are mainl caused due to #roken wire conditions" and these loads have much more effect on the design of the tower than an other load%
The un#alanced &ull due to #roken conductor" in case of su&&orts with sus&ension strings" ma #e assumed e7ual to /- &er cent of the ma$imum working tension +Tc0 of the conductor%
In case of #undle conductors" the &ull due to #roken conductor ma #e assumed to #e e7ual to
2or the ground wire #roken condition" ,-- &er cent or such &ercentage of ground! wire tension" for which the ground!wire clam& is &ro&ortioned and whichever is less should #e considered for the &ur&ose of design of tower%
Thus" if Te and Tc are the working tensions in earth wire and conductor res&ectivel" and K is the deviation" the longitudinal loads will #e given as (ON:ITUDINA( (OADS
TRANSMISSION (IN* TO=*RS Torsional loads4 +Mt0 Torsional moment is caused under #roken wire condition" when the #roken earth wire or conductor wire is located at an eccentricit e with res&ect to the centre line of the tower% The torsional moment is given as Mt F '( e
The torsional shear &er face" 8t F mt 8<#
TRANSMISSION (IN* TO=*RS *22*CT O2 T*M'*RATUR* 9ARIATION Temperature variations:
The tem&erature range varies for different localities under different diurnal and seasonal conditions% The a#solute ma$imum and minimum tem&eratures" which ma #e e$&ected in different localities in the countr are indicated on the ma&s of India in 2ig L ) ,-" res&ectivel%
These ma #e used for assessing the tem&erature stresses on conductors and ground wires% The a#solute ma$imum tem&erature values given in figure shall #e increased # a#out ,Eo C to allow for the suns radiation" heating effect of current" etc%" in the conductor%
Effect of temperature on cable tension
A conductor hangs freel #etween two su&&orts +towers0 at the ends% It is therefore su#6ected to tension T% The value of ca#le tension T de&ends u&on the tem&erature%
The tension in conductor #ecomes +i%e% Tma$%0 when the atmos&heric tem&erature t is minimum +tmin 0% This ma$imum tem&erature should not e$ceed the allowa#le tension in the conductor%
Similarl" the tension in the conductor #ecomes minimum +i%e% T min0 when the atmos&heric tem&erature t is +i%e% tma$0% ;ence the resulting tensile stress in the conductor is tem&erature de&endent%
2ig% L Ma& showing highest ma$imum tem&erature
2ig% ,- Ma& showing lowest minimum tem&erature
TRANSMISSION (IN* TO=*RS ANA(?SIS AND D*SI:N Analysis of tower A transmission line tower is a three!dimensional cantilever truss% Its analsis as a s&ace frame is highl tedious% ;owever" a ma6orit of the forces acts onl at its to& end% The conventional &rocedure is to anal3e it # resolving the tower in &lanar frames% 2ig ,, shows various situations of hori3ontal load '
2ig% ,, (ongitudinal load on tower
TRANSMISSION (IN* TO=*RS ANA(?SIS AND D*SI:N Design of members The mem#ers of the tower are either tension mem#ers or com&ression mem#ers% Since the mem#ers are slender" secondar stresses are ignored% The design of towers are done as &er recommendations contained in IS 4 -< ! ,LEE% Some of the salient recommendations are given here% i.
actors of safety: In accordance with Rule E@+,0 +a0 of Indian *lectricit Rules ,L/@" the factor of safet +n0 in the design of structural mem#ers of steel transmission line towers ma #e assumed as <%- under normal conditions and ,%/ under #roken!wire conditions% In accordance with Rule E@+,0 +c0 of the Indian *lectricit Rules ,L/@" the factors of safet of conductor and ground wires ma #e assumed as s&ecified #elow%
The minimum factor of safet +n0 for conductors ma #e assumed as <%- #ased on their tensile strength at minimum tem&erature and ma$imum wind &ressure e$&ected in the region% In addition" the conductor tension at 1<-C without e$ternal load" should not e$ceed the following &ercentages of the ultimate tensile strength of the conductors4 a%
Initial unloaded tension 4 1/ &er cent%
#%
2inal unloaded tension 4
TRANSMISSION (IN* TO=*RS ANA(?SIS
ii.
AND D*SI:N
Allowable stress: The allowa#le stresses given here are #ased on recommendation contained in IS 4 -< ! ,LEE" using the factors of safet +n0 s&ecified a#ove" for steel in general +having ield stress f 0 and for steel to IS 4 <<@ having f F
2or tension mem#ers4 f at F f 8 n
#%
2ore com&ression mem#ers4
=here"
for l8r Cc "
f ac F +f 8 n0
for l8r Cc "
f acF ,%L@ e -@ 8 +n +l8r0<0
Cc F ,<.%. for mild steel l F effective length r F minimum radius of gration
>ased on a#ove e7uations" the values of 2ac for steel are given in Ta#le%
'*RMISSI>(* STR*SS f ac IN COM'R*SSION AS '*R IS 4 -<
TRANSMISSION (IN* TO=*RS ANA(?SIS AND D*SI:N Slenderness ratio IS 4 -< s&ecifies the following limiting values of l8r ratio where ( is the actual length of the mem#er" #etween the centres of end connections% (IMITIN: 9A(U*S O2 (8r
!ermissible stresses in bolts: The 6oints of tower are made # using #olts" to facilitate eas an 7uick installation% The following are the &ermissi#le stresses4 ,%
'ermissi#le tensile stress on root of thread 4 ,L.8n N8mm <
<%
'ermissi#le shearing stress on gross area of #olt 4 <,8n N8mm <
1%
'ermissi#le #earing stress on gross area of #olt 4 .1@8n N8mm <
=here" n F <%- for normal condition and ,%/ for #roken condition%
2OUNDATION 2OR TO=*RS
The sta#ilit of a tower de&ends #oth on the strength as well as sta#ilit of foundations%
The foundation for a tower is designed for the following forces8momenls
a0
Downward load on the leg
#0
U&lift load on leg
c0
;ori3ontal thrust
d0
Over turning moments
)
:enerall% the load acting on the to& of a footing is inclined" and this Inclined load can resolved into vertical and hori3ontal +or lateral0 com&onents%
)
The lateral and longitudinal loads" acting at a great height cause large overturning moments% which are to #e resisted # the foundation with a minimum factor of safet of three% 2OUNDATION 2OR TO=*RS
